std::condition_variable::notify_one/all() should be called after unlocking mutex (#1448)

* Move next job in task queue rather than copy

* Notify waiting thread after unlocking mutex

* Add unit test for TaskQueue

* Don't use C++14 feature in test code

httplib.h

@@ -549,8 +549,11 @@ public:
   ~ThreadPool() override = default;
 
   void enqueue(std::function<void()> fn) override {
-    std::unique_lock<std::mutex> lock(mutex_);
-    jobs_.push_back(std::move(fn));
+    {
+      std::unique_lock<std::mutex> lock(mutex_);
+      jobs_.push_back(std::move(fn));
+    }
+
     cond_.notify_one();
   }
 
@@ -559,9 +562,10 @@ public:
     {
       std::unique_lock<std::mutex> lock(mutex_);
       shutdown_ = true;
-      cond_.notify_all();
     }
 
+    cond_.notify_all();
+
     // Join...
     for (auto &t : threads_) {
       t.join();
@@ -583,7 +587,7 @@ private:
 
           if (pool_.shutdown_ && pool_.jobs_.empty()) { break; }
 
-          fn = pool_.jobs_.front();
+          fn = std::move(pool_.jobs_.front());
           pool_.jobs_.pop_front();
         }
 

test/test.cc

@@ -5,6 +5,7 @@
 #include <atomic>
 #include <chrono>
 #include <future>
+#include <memory>
 #include <sstream>
 #include <stdexcept>
 #include <thread>
@@ -5522,3 +5523,18 @@ TEST(SocketStream, is_writable_INET) {
   ASSERT_EQ(0, close(disconnected_svr_sock));
 }
 #endif // #ifndef _WIN32
+
+TEST(TaskQueueTest, IncreaseAtomicInteger) {
+  static constexpr unsigned int number_of_task{1000000};
+  std::atomic_uint count{0};
+  std::unique_ptr<TaskQueue> task_queue{
+      new ThreadPool{CPPHTTPLIB_THREAD_POOL_COUNT}};
+
+  for (unsigned int i = 0; i < number_of_task; ++i) {
+    task_queue->enqueue(
+        [&count] { count.fetch_add(1, std::memory_order_relaxed); });
+  }
+
+  EXPECT_NO_THROW(task_queue->shutdown());
+  EXPECT_EQ(number_of_task, count.load());
+}